A prior art rotary hammerdrill bit for use with a percussion or hammer drilling device (DE 38 25 107 C2) has a main cutting plate that projects over the diameter of the drill head by a minimal amount. Cutting pins are arranged on a diameter on both sides of the main cutting plate, and thus are positioned in the same way relative to each drill blade of the main cutting plate. Both drill blades of the main cutting plate have the same completely concave contour in normal projection to the plate plane. The free front ends of the cutting pins serve to surface chiseling of the drill hole base, so that each following drill blade of the main cutting plate can operate more effectively.
Quiet operation and adhering to precise drill geometry has already been known to be problematic with the prior art rotary hammerdrill bit. This may have something to do with the fully concave trajectory of both drill blades of the main cutting plate.
It has already been proposed to design a rotary hammerdrill bit asymmetrically with cutting pins which are arranged on both sides of a main cutting plate substantially symmetrically at an angle of less than 90° of the main cutting axis defined by the main cutting plate (EP 0 943 780 A2). Quiet operation and precise drill geometry are achieved here in a better way due to the fact that the drill blades of the main cutting plate are extending substantially roof-shaped at an incline radially from the tip. In contrast to the above discussed prior art there is no concave contour of the drill blades of the main cutting plate.
The cutting pins of above mentioned prior art (EP 0 943 780 A2) are arranged on the drill head in the same essentially radial positions with the rotation surfaces defined by those cutting pins lying slightly below the rotation surface defined by the drill blades as such as seen in the advance direction of the drill bit.
A problem with above described further prior art, however, is the power necessary to achieve a specific drill speed. The two drill blades of the main cutting plate are contacting the concrete in full length and therefore need the highest possible torque. This is only slightly reduced by the fact that in percussion drilling and hammer drilling the movement of the drill head in the advance direction leads to a contact of the cutting pins with the concrete in the drill hole at persistently varying angle positions.
Further prior art (EP 0 824 626 B1) shows a drilling tool similar to the drilling tool discussed above but with secondary cutting plates instead of the cutting pins. Here the length of the main cutting plate is supplemented by the lengths of the secondary cutting plates with the result that the torque necessary for this drill bit is more or less the same as in the prior art mentioned before.